The present invention relates to a white powder and a process for producing the same. More particularly, the invention relates to a white powder which has higher lightness (whiteness) than conventional ones and is usable for a variety of purposes such as color inks, color fillers for plastics/paper, color toners, color inks for ink-jet printers, inks and toners for forgery prevention, general coating compositions, powder pigments/coating compositions for motor vehicles, coating compositions for electrostatic coating, pigments for cosmetics, pigments for art objects such as craftworks and ceramic art objects, pigments for (to be deposited on) fibers, pigments (especially for magnetic shielding) and fillers for decorative papers/decorative sheets, catalytic coating compositions, and heat-resistant coating compositions, and to a process for producing the same.
A technique is known which comprises coating a powder with another substance to impart a new function thereto in order to use the powder in various applications.
For example, a conventional, one-component type, magnetic color toner or magnetic color ink is produced by forming a colored layer on base particles having magnetics, e.g., an iron powder.
For obtaining a clear color image with this one-component type magnetic color toner or magnetic color ink, it is necessary to color the magnetic toner or ink itself in a bright tint. However, even when a colored layer is formed directly on the surface of the magnetic particles serving as a base, the coated particles as a whole assume a dark color because the magnetic particles are generally black.
In order to overcome the drawback, the following have been proposed: a technique in which a metal film is formed on base particles to make the powder white based on the reflective function of the film (Unexamined Published Japanese Patent Applications Nos. 3-271376 and 3-274278); a technique which comprises dispersing base particles into a metal alkoxide solution and hydrolyzing the metal alkoxide to thereby form a metal oxide film having an even thickness of from 0.01 to 20 xcexcm on the surface of the base particles (Unexamined Published Japanese Patent Application No. 6-228604); a functional powder having on the surface thereof thin films of a metal oxide arranged alternately with thin films of a metal (Unexamined Published Japanese Patent Application No. 7-90310); and a technique for producing a powder having a denser and stabler multilayered metal oxide film which comprises heat-treating a powder coated with a multilayered metal oxide film (International Publication WO 96/28269).
In particular, in the case of the powders described above having two or more layers of a metal oxide film or metal film, a special function can be imparted thereto by relating the thickness of each layer. For example, when coating films differing in refractive index are formed on the surface of base particles each in a thickness corresponding to one-fourth the wavelength of an incident light, a white powder can be produced which reflects all the incident light. It is suggested that the white powder thus obtained can be used to obtain a white magnetic toner or ink, and that a color magnetic toner or ink colored in a bright tint can be produced by further forming a colored layer on the surface of the white powder.
However, the technique described in Unexamined Published Japanese Patent Applications Nos. 3-271376 and 3-274278, in which a metal film is formed, has the following drawbacks. The reflectance of a powder can be heightened to the reflectance inherent in the metal by increasing the number of films or film thickness and the powder can be thus whitened. However, a higher degree of whiteness cannot be expected after the number of films or the film thickness has reached to a certain degree. In addition, the whiteness obtained is insufficient.
Furthermore, in the techniques described in Unexamined Published Japanese Patent Applications Nos. 6-228604 and 7-90310 and International Publication WO 96/28269, the larger the number of films or film thickness, the higher the reflectance and, hence, the higher the whiteness. Thus, the properties of the film are enhanced. However, the larger the number of films or film thickness, the more the properties of the base particles are diminished. For example, in the case where a magnetic powder is used as base particles, magnetism becomes lower as the number of films or the film thickness increases.
In other words, the following can be said. In the white powders obtained by the techniques described above, it is necessary to reduce the number of films or the film thickness for taking advantage of properties possessed by the base particles. However, there has been a fear that the desired whiteness may not be obtained when the number of films or the film thickness is reduced.
Accordingly, an object of the invention is to overcome the drawbacks of the conventional techniques described above and to provide a white powder which has high whiteness while retaining properties of the base particles, more specifically one which has high whiteness even when it has a relatively small number of films with a relatively small thickness so as to take advantage of properties of the base particles. Another object of the invention is to provide a process for producing the white powder.
The present inventors made intensive investigations. As a result, they have found that a film-coated powder which has been whitened based on scattering reflection can be obtained by forming at least one coating layer comprising a crystallized-fine-particle aggregate comprising crystallized fine particles and having voids among the crystallized fine particles. Thus, the inventors have succeeded in accomplishing the objects described above.
Namely, the invention relates to the following (1) to (20).
(1) A white powder characterized by comprising base particles having on the surface thereof at least one coating film comprising a crystallized particle aggregate which is capable of imparting a white color based on the scattering reflection of light and which comprises crystallized particles and has voids among the crystallized particles.
(2) A white powder characterized in that it comprises base particles having on the surface thereof at least one coating film comprising a crystallized-particle aggregate which comprises crystallized particles and has voids among the crystallized particles, and that a white color is imparted thereto based on the scattering reflection of light occurring between the surface of the crystallized particles and the voids.
(3) The white powder as described in (1) or (2) above, characterized in that the crystallized particles are ones irregular in particle diameter.
(4) The white powder as described in (1) or (2) above, characterized in that the coating film is a multilayered film.
(5) The white powder as described in (1) or (2) above, characterized by having, on the surface of the coating film, a coating film comprising particles capable of filling up the voids present in that surface.
(6) The white powder as described in (1) or (2) above, characterized in that the coating film is a high-refractive-index film.
(7) The white powder as described in (5) above, characterized in that the coating film comprising particles capable of filling up the voids present in that surface is a silica film or a titania film.
(8) The white powder as described in (1) or (2) above, characterized in that the coating film is one formed by forming solid-phase particles in an aqueous solution to coat the base particles with the solid-phase particles and then burning the coated base particles.
(9) The white powder as described in (8) above, characterized in that before the burning is conducted, the coating layer is coated with particles capable of constituting a film which fills up the voids present in the surface of the coating layer.
(10) The white powder as described in (1) or (2) above, characterized in that the coating layer is one formed by adhering crystallized particles to the surface of a base powder in a liquid containing the crystallized particles and the base powder dispersed therein.
(11) A process for producing a white powder, characterized by coating the surface of base particles with at least one coating film comprising a crystallized-particle aggregate which is capable of imparting a white color based on the scattering reflection of light and which comprises crystallized particles and has voids among the crystallized particles.
(12) A process for producing a white powder, characterized by coating the surface of base particles with at least one coating film comprising a crystallized-particle aggregate which comprises crystallized particles and has voids among the crystallized particles to thereby impart a white color thereto based on the scattering reflection of light occurring between the surface of the crystallized particles and the voids.
(13) The process as described in (11) or (12) above, characterized in that the crystallized particles are ones irregular in particle diameter.
(14) The process as described in (11) or (12) above, characterized in that the coating film is a multilayered film.
(15) The process as described in (11) or (12) above, characterized by coating the surface of the coating layer with a coating film comprising particles capable of filling up the voids present in that surface.
(16) The process as described in (11) or (12) above, characterized in that the coating film is a high-refractive-index film.
(17) The process as described in (15) above, characterized in that the coating film comprising particles capable of filling up the voids present in that surface is a silica film or a titania film.
(18) The process as described in (11) or (12) above, characterized by forming solid-phase particles in an aqueous solution to coat the base particles with the solid-phase particles and then burning the coated base particles to thereby form the coating film.
(19) The process as described in (18) above, characterized in that before the burning is conducted, the coating film is coated with particles capable of constituting a film which fills up the voids present in the surface of the coating film.
(20) The process as described in (11) or (12) above, characterized in that the coating layer is one formed by adhering crystallized particles to the surface of a base powder in a liquid containing the crystallized particles and the base powder dispersed therein.